114 DE. 0. WARD ON SLICKENSIDES OE EOCK>STEIATIONS. 



gical to assign a different reason for them and those in the chalk, 

 and hence I venture to include all striated and polished surfaces 

 under the general term of slickensides. The surfaces of the striated 

 rocks are harder than the chalk within, I presume, from the pressure 

 and condensation ; but the fragments in the angles when fresh are 

 very friable, splitting with the greatest ease in the lines of striation, 

 and each face is also striated. Some of the faults in the chalk- 

 marl are filled with compressed fragments, each surface of which is 

 striated ; and the contorted strata are similarly marked. In this 

 neighbourhood, at least, I am convinced that striation is always 

 attended with disturbance of the strata; and I venture to say that 

 neither the coralline nor the crystalline theory is supported by the 

 microscope, which has never revealed zoophytes or crystals, except 

 where slickensides have been moulded on the sides of fissures — and 

 that " cone in cone" is a substantial structure, while striations are 

 entirely superficial. The concretionary chalk, of which Mr. Fordham 

 has kindly sent me specimens, is common here, where it is called 

 " hommy mommy" by the workmen. It is much harder than the 

 chalk or chalk-marl, and is therefore rejected for lime-burning. It 

 is not so often striated as the other forms of chalk. 



Discussion. 



Mr. Whitakee said that some of the specimens on the table sent 

 by Dr. Ward showed slickensides, but that in certain eases the 

 fibrous structure runs through the whole substance. 



Mr. "W. T. Blanford stated that he had seen a similar structure 

 in an undisturbed white clay resembling pipeclay; so that he thought 

 the appearance could not be produced by motion. 



Mr. Evans remarked that the structures brought forward appeared 

 to be of two kinds : — first, slickensides pure and simple; and secondly, 

 fibrous structure, — the latter probably due to chemical action, either 

 by an incipient formation of aragonite, or by the formation of that 

 mineral and its subsequent partial decomposition. 



